Drought stress is the main cause of crop yield loss worldwide, and as global temperatures rise and rainfall patterns change, water availability will likely remain the major challenge facing crop yields around the world. What comes to mind when thinking about drought stress is how it causes plants to become a desiccated shell of their former self; unable to accumulate enough water to fuel key metabolic processes such as photosynthesis, respiration and growth. Much research has focussed on such processes, hoping to improve their efficiency and output under challenging growth conditions. However, an emerging field of research is now focussing on a seemingly less obvious consequence of drought stress; reduced nutrient uptake from the soil.
Nutrients such as nitrogen, phosphorus, zinc and iron take many forms in soil; largely being released after degradation of organic material (such as leaves and manure etc.) by the soil’s indigenous microbial population. Nitrogen and phosphorus are required for photosynthesis as well as the synthesis of amino acids and DNA molecules; the building blocks of life. These macronutrients are subsequently the major components of chemical fertilizers, as deficiencies in them can have major impacts of plant health and crop yields. Micronutrients such as zinc and iron are needed in smaller quantities but play similarly crucial roles in crop growth, as they are required in photosynthesis, RNA synthesis and efficient enzyme catalysis. As well as being essential for the growth of the plant, these nutrients are also essential for our own health as macronutrients are used for DNA and protein synthesis as well as carbohydrate and fat metabolism, whilst micronutrients are used for haemoglobin production, hormone synthesis, wound healing and efficient cell division and growth. This means that food crops must have high levels of these nutrients if they are to be considered truly nutritious.
Research in the last decade has shown that drought stress not only inhibits the growth and yield of key crops such as wheat, rice and maize, but it also reduces nutrient concentration within these plants. One study found that nitrogen and phosphorus content decreased in barley and maize after a period of drought stress. Another study saw similar results, as the amount of phosphorus and potassium uptake in maize decreased as the severity of drought stress increased. Similar patterns have been seen for micronutrient uptake, as experiments in chamomile tea plants showed that uptake of iron and zinc were reduced when drought stress was administered at certain developmental stages. It is thought that because water uptake from the soil into the plant is reduced under drought stress, the uptake of soluble nutrients (dissolved in the water) is also diminished. It is almost as if plants know this will be the case, as many studies have shown that plants instinctively produce more proteins involved in nutrient uptake when they sense any sign of water shortage; seemingly an effort to maximise the amount of nutrient uptake from the limited amounts of soil moisture available.
As well as stifling nutrient uptake via reduced water flow into the plant, drought stress has also been shown to reduce the availability of key nutrients within the soil, as a result of reduced microbial degradation activity. One study found that carbon and nitrogen decomposition decreased by more than 50% under drought stress, whilst another study found that more frequent periods of drought led to reduced carbon and nitrogen concentrations in soil. This spells a large problem for plants trying to accumulate nutrients under drought stress, as not only is the nutrient-carrying water less abundant, but this water also contains fewer of the prized nutrients.
This research sheds light on a lesser-known consequence of drought stress; a consequence that must be taken seriously, as billions of people around the world rely on crops such as wheat, rice and maize for the majority of their nutrient intake, as well as their caloric intake. Over 2 billion people are already deficient in at least one micronutrient, and as periods of drought stress become more common worldwide, this research tells us that this number will likely rise. Therefore, the development of high yielding, nutritious and drought tolerant crop varieties are crucial if we are to avoid famines and diseases associated with reduced crop yields and nutrient contents caused by drought stress.